Pregnancy can seem strange and mysterious. If you’ve recently found out you’re pregnant, you might want to know what is happening inside you as your baby grows. Here we’ll take a look at the growth and development of your pregnancy week by week, so you can learn about and enjoy the wonderful changes that occur in every pregnancy.

When Does Pregnancy Start?

Before learning about how pregnancy progresses, it’s important to understand when and how it starts. You may be surprised to learn there are two ways of dating a pregnancy: gestational age and fertilization age.

Gestational age refers to the amount of time since the first day of the mother’s last menstrual period. Most doctors will refer to the age of your baby in terms of gestational age. In most cases, the gestational age begins two or more weeks before the fertilization of the egg by sperm.

Fertilization age refers to how long it has been since the actual event of conception: the fertilization of an egg by sperm. Because most women ovulate (release an egg into their womb from the ovary) in the middle or last third of their menstrual cycle, fertilization age is typically about seven to 14 days later than the gestational age. However, every woman’s cycle is different and thus fertilization age is slightly less accurate than gestational age.

For purposes of convenience, the dates in this development guide use gestational age as a milestone. To determine when these milestones occur in terms of fertilization age, subtract approximately two weeks from the dates in this guide.

The Stages of Pregnancy

Two Weeks After Your Period (Conception Day): The egg and sperm come together, most often in the fallopian tubes, to form a single cell known as a zygote. The zygote contains all of the genetic information present in an adult, including height, eye color, hair color, gender and more.

As the single-celled zygote divides and grows, it becomes what we call an embryo. As the embryo gets larger it travels down the fallopian tube and eventually reaches the uterus about three or four days after conception. At this point, the embryo is about the size of a grain of sand.

Three Weeks: By the sixth or seventh day of pregnancy the embryo implants itself into the lining of the uterus, triggering pregnancy responses in the mother. At this point, blood tests can show that the mother is pregnant.

Four Weeks: By one month after the mother’s last period, the embryo is now fully connected to the mother’s bloodstream via the uterus. At this point, urine tests can detect pregnancy hormones accurately.

Five Weeks: The embryo is developing rapidly and beginning to form internal organs. The heart is the first to begin functioning, and it starts to beat at about this point. Other organ systems including the brain, lungs, and stomach, are beginning to form as well.

Six Weeks: The embryo is now about ⅛ of an inch in size, about the size of a large peppercorn. The brain, spinal cord, and central nervous system is almost fully formed, and the eyes, arms, and legs are visibly developing. The embryo’s heart beats about 120 times every minute - twice per second - at this stage of pregnancy.

Seven Weeks: The baby’s circulatory system is almost fully formed, and it has begun to create its own blood inside it’s body. Depending on whether the embryo is male or female, it is beginning to form either testicles or ovaries.

Eight Weeks: The embryo is now nearly ½ an inch in size, slightly smaller than a U.S. dime. Fingers, toes, elbows, and knees are forming on the limbs, and the embryo is able to move them in response to touch by reflex. The embryo’s lungs, tongue, and tooth buds are also forming at this pregnancy stage.

Nine Weeks: The development of the pregnancy becomes more complex at this point. By nine weeks, the embryo is ¾ of an inch in size, about as big as a penny. Limbs, fingers, and toes continue to grow and are beginning to form hard bones inside. The eyes, ears, and nose are also forming rapidly, and the eyes even begin to take on color in the retina.

Ten Weeks: By this stage of pregnancy the embryo is rapidly expanding its nervous system, limbs, and internal and external organs. The baby can begin purposely moving its arms and legs.

Eleven Weeks: After about three months of pregnancy, we no longer refer to the growing baby as an embryo. Instead we use the Latin word fetus, which means “young one.” At this stage the fetus has all of its major organ systems in place and is beginning to look recognizably like a baby. It is about one and a half to two inches long - around the same size as a lime. The fetus can perform many familiar motions including yawning, sucking, and stretching. The eyelids are developed and close to protect the eyes, and the kidneys begin to produce urine.

At this point, the baby’s internal organ systems are almost fully developed, and its external appearance is changing rapidly as it grows larger. Fetal development by week is at this point mostly focused on growth.

Fourteen Weeks: The fetus is about three inches long, somewhere around the size of a peach. It is growing fingernails and toenails, and the fetus can usually find its own thumb and suck on it in the womb.

Sixteen Weeks: By four months, the fetus is about five inches long and weighs about four ounces - about the size and weight of a large avocado. The heart is beating very fast at this stage of pregnancy: about 110 to 180 times per minute (three times every second!). Ultrasounds at this stage can often determine the gender of the fetus.

Eighteen Weeks: The fetus is now nearly six inches long and approximately seven ounces. The baby’s internal development continues, with the skeleton growing and hardening. External development is also progressing well: the baby can blink and frown, and the fingers and toes have developed the prints they will have for life.

Twenty Weeks: You may be feeling the fetus move inside you regularly now, as it can easily coordinate its arm and leg movements to squirm and kick. It is about ten inches long and weighs nearly 11 ounces, about the size of a pomegranate. The baby is developing sleeping patterns, and some studies suggest it can also respond to pain signals. This pregnancy stage also marks the approximate “halfway point” for the pregnancy.

Twenty-two Weeks: The fetus is about 12 to 14 inches long from head to heel, and weighs almost a pound (16 ounces). Hair is beginning to develop on the head and skin.

Twenty-four Weeks: The fetus weighs approximately 1.5 pounds (22 to 24 ounces) and is about the size of a head of lettuce. The face is fully formed by this stage, and it has begun to practice breathing by inhaling and exhaling amniotic fluid. Most fetuses at this point can also hear voices and other sounds from outside the womb, and it may begin to recognize your voice, breathing, and heartbeat. They can even be “startled” by noises applied to the mother’s abdomen.

Twenty-six Weeks: Fetal development is beginning to reach the final stages as the mother enters her third trimester. The fetus weighs about two pounds and is coming up to 16 inches in length, about the size of a large eggplant. The eyes are almost fully-formed and can respond to light, and the teeth buds are permanently in place for baby teeth to start growing. The baby is also growing eyebrows and eyelashes.

Twenty-eight weeks: With the support of intensive care, many babies born at this stage stand a good chance of survival. The brain is developing the ability to regulate breathing and body temperature, and the immune system is starting to support itself. The most important stages of fetal development are finished at this point - from here on, the baby mostly just grows larger.

Thirty-four Weeks: Fetal development is nearly finished, and the baby is preparing to be born. Most babies are extremely active in the womb by this point, and have developed reflexes including responses to light, sound, and touch. The lungs are still in their final stages of development, so babies born at this stage may still need some assistance breathing. On average, babies at this development stage are about 17 inches long and nearly five pounds in weight.

Forty Weeks: On average a baby at this stage is nearly 20 inches in length and weighs seven or eight pounds (though some can be significantly bigger). Most babies naturally position themselves head-down in the uterus and start to migrate down toward the pelvis in preparation for birth.

Forty weeks is also the typical “due date” for a pregnancy. However, only about four percent of babies are actually born on their due dates. It’s not uncommon for pregnancies to go on a week or two after the due date. Your prenatal doctor may even recommend inducing pregnancy to make sure the baby doesn’t grow too large inside your womb.

It’s important to remember that every pregnancy is different, and your week by week experience may be different from that of other women. However, one thing is certain - when you find out you’re pregnant, you should schedule an appointment with a prenatal care doctor who can advise you on the best ways to care for your developing child. A doctor can also answer all your questions and give you the best advice for the remainder of your pregnancy.

Whether you’re excited, unsure or scared about your pregnancy, you should call our office to speak with one of our compassionate and professional nurses here at the Pregnancy Care Center of Grants Pass. Our medical clinic is AAAHC accredited and we have years of experience helping women through their pregnancy in providing free pregnancy related medical services and support along with a wealth of information and resources so you will feel equipped and prepared. Call (541) 479-6264 today to schedule an appointment or stop by our clinic at your convenience.